130
Baumann et al.
sediments is probably due to its broad range of
ecological tolerance.
The progressive increase in abundance of C.
leptoporus with increasing water depth may again
be due to the resistance of this species against dissolution (see Dittert et al. this volume). Giraudeau
(1992) and Giraudeau and Rogers (1994) also suggested that the growth of this species is optimal in
oligotrophic conditions west ofthe main Benguela
upwelling. However, this is neither really confirmed
by its surface-water distribution pattern nor by its
absolute numbers in the surface sediments (Fig. 8;
Giraudeau and Bailey 1995). In addition, correlating C. leptoporus to oligotrophic conditions, as it
is done for the southern Benguela complement
(Giraudeau 1992; Giraudeau and Rogers 1994), is
not applicable due to different findings in the equatorial Atlantic (Kinkel et al. in press). The conditions in waters of both equatorial upwelling and
off the coast of SW -Africa are possibly more
mesotrophic than fully eutrophic or fully oligotrophic and, thus, the areas are comparable. Also,
different morpho types of C. leptoporus have been
recognized (McIntyre et al. 1970; Knappertsbusch
et al. 1997), from which at least one is an
eurythermal species, and the other favors warm
tropical and subtropical waters. Their occurrences
may not only depend on the temperature of the
surface-waters, but also on other environmental
conditions such as content of nutrients. Differences
in morphotype characteristics, such as width and
number of elements forming the placolith shield,
however, have not been distinguished in the present
study.
Gephyrocapsa oceanica is found to be less
important than described by Giraudeau (1992) from
the uppermost slope. This may be due to different
counting and preparation techniques, and especially
based on the fact that Giraudeau (1992) excluded
three dominant taxa (e.g. E. huxleyi) from quantitative analysis. G. oceanica has been interpreted
to prefer waters of high fertility and neritic environments (Mitchell-Innes and Winter 1987;
Fincham and Winter 1989), and has been reported
to bloom in upwelled waters of low latitudes
(Kleijne et al. 1989).
Other species make a minor but consistent contribution to the assemblages in all of the surface
sediments, but they constitute less than 5% of all
species. Thus, their distribution is more diffuse than
the pattern of the prominent species. Nevertheless,
the coccolith distribution in the studied area seems
to be closely related to the combination of the Be
and the upwelling of cold, nutrient-rich subsurfacewater off Namibia. In addition, a correlation
between the high productivity in the upwelling area
and the carbonate production by coccoliths
seems to be reasonable, although any dilution with
terrigenous material or processes of dissolution may
alter this relationship.
Primary Productivity Reconstructions in the
Equatorial Atlantic
While former studies generally considered
coccolith abundances as a proxy for certain oceanographic conditions, the use of quantitative analysis enables a discussion on how the coccolithophore
assemblage responds to changing oceanographic
conditions in terms of productivity. Until recently
only few studies (Backman and Shackleton 1983;
Gard 1989; Henrikson 1996; Flores et al. 1997)
demonstrated, that quantitative and semi-quantitative analyses of coccolith assemblages can be used
for productivity reconstructions in other regions and
on different time scales. Phytoplankton productivity in general is controlled by the availability of
nutrients, light and temperature (Winter and Siesser
1994). Since light and temperature are not limiting
in the equatorial Atlantic, changes in productivity
are triggered by the amount of nutrients available
in the euphotic zone. A dilution effect caused
by terrigenous input can be neglected in the
equatorial Atlantic. Therefore, fluctuations of
the coccolith accumulation rate can be used
to monitor coccolithophore production. Using
coccolithophores as a productivity proxy provides
further information for discussing other existing
paleoproductivity estimations (e.g. Muller and
Suess 1979; Mix 1989; Brummer and van Eijden
1992; Sarnthein et al. 1992; Struck et al. 1993;
Ruhlemann et al. 1996), that all have certain
inaccuracies. Among all phytoplankton groups
coccolithophores may have the best fossil record,
as silicate or organic walled microfossils usually are
Baumann et al.
sediments is probably due to its broad range of
ecological tolerance.
The progressive increase in abundance of C.
leptoporus with increasing water depth may again
be due to the resistance of this species against dissolution (see Dittert et al. this volume). Giraudeau
(1992) and Giraudeau and Rogers (1994) also suggested that the growth of this species is optimal in
oligotrophic conditions west ofthe main Benguela
upwelling. However, this is neither really confirmed
by its surface-water distribution pattern nor by its
absolute numbers in the surface sediments (Fig. 8;
Giraudeau and Bailey 1995). In addition, correlating C. leptoporus to oligotrophic conditions, as it
is done for the southern Benguela complement
(Giraudeau 1992; Giraudeau and Rogers 1994), is
not applicable due to different findings in the equatorial Atlantic (Kinkel et al. in press). The conditions in waters of both equatorial upwelling and
off the coast of SW -Africa are possibly more
mesotrophic than fully eutrophic or fully oligotrophic and, thus, the areas are comparable. Also,
different morpho types of C. leptoporus have been
recognized (McIntyre et al. 1970; Knappertsbusch
et al. 1997), from which at least one is an
eurythermal species, and the other favors warm
tropical and subtropical waters. Their occurrences
may not only depend on the temperature of the
surface-waters, but also on other environmental
conditions such as content of nutrients. Differences
in morphotype characteristics, such as width and
number of elements forming the placolith shield,
however, have not been distinguished in the present
study.
Gephyrocapsa oceanica is found to be less
important than described by Giraudeau (1992) from
the uppermost slope. This may be due to different
counting and preparation techniques, and especially
based on the fact that Giraudeau (1992) excluded
three dominant taxa (e.g. E. huxleyi) from quantitative analysis. G. oceanica has been interpreted
to prefer waters of high fertility and neritic environments (Mitchell-Innes and Winter 1987;
Fincham and Winter 1989), and has been reported
to bloom in upwelled waters of low latitudes
(Kleijne et al. 1989).
Other species make a minor but consistent contribution to the assemblages in all of the surface
sediments, but they constitute less than 5% of all
species. Thus, their distribution is more diffuse than
the pattern of the prominent species. Nevertheless,
the coccolith distribution in the studied area seems
to be closely related to the combination of the Be
and the upwelling of cold, nutrient-rich subsurfacewater off Namibia. In addition, a correlation
between the high productivity in the upwelling area
and the carbonate production by coccoliths
seems to be reasonable, although any dilution with
terrigenous material or processes of dissolution may
alter this relationship.
Primary Productivity Reconstructions in the
Equatorial Atlantic
While former studies generally considered
coccolith abundances as a proxy for certain oceanographic conditions, the use of quantitative analysis enables a discussion on how the coccolithophore
assemblage responds to changing oceanographic
conditions in terms of productivity. Until recently
only few studies (Backman and Shackleton 1983;
Gard 1989; Henrikson 1996; Flores et al. 1997)
demonstrated, that quantitative and semi-quantitative analyses of coccolith assemblages can be used
for productivity reconstructions in other regions and
on different time scales. Phytoplankton productivity in general is controlled by the availability of
nutrients, light and temperature (Winter and Siesser
1994). Since light and temperature are not limiting
in the equatorial Atlantic, changes in productivity
are triggered by the amount of nutrients available
in the euphotic zone. A dilution effect caused
by terrigenous input can be neglected in the
equatorial Atlantic. Therefore, fluctuations of
the coccolith accumulation rate can be used
to monitor coccolithophore production. Using
coccolithophores as a productivity proxy provides
further information for discussing other existing
paleoproductivity estimations (e.g. Muller and
Suess 1979; Mix 1989; Brummer and van Eijden
1992; Sarnthein et al. 1992; Struck et al. 1993;
Ruhlemann et al. 1996), that all have certain
inaccuracies. Among all phytoplankton groups
coccolithophores may have the best fossil record,
as silicate or organic walled microfossils usually are